FIG. 10 shows a conventional antenna device, for example, as disclosed in "Inverted F-shaped Antenna for Portable Radios", Hiroshi HARUKI et al., No. 613 of 1982 Overall Annual Conference of the Institute of Electronics and Communication Engineers of Japan.
In FIG. 10, the conventional inverted F antenna 17 includes a plate conductor 11 with an approximately rectangular shape constituting a radiative conductor, and a ground conductor 12 constituted by metallic plates of a radio frame body. The plate conductor 11 is arranged approximately in parallel with the ground conductor 12, and one end in its longitudinal direction is connected to the ground conductor 12 by a grounding conductor 13. A coaxial line 14 as a feeder line for the antenna 17 is comprised of an internal conductor 15 and an external conductor 16.
The internal conductor 15 of the coaxial line 14 is directly connected to the facing surface of the plate conductor 11 in a position close to the grounding conductor 13 by soldering or the like. The external conductor 16 of the coaxial line 14 is connected to the ground conductor 12.
Next, the operation of the conventional inverted F antenna 17 will now be described.
In the inverted F antenna 17, the plate conductor 11 and the ground conductor 12 constitute a parallel flat line, and, since one end in the longitudinal direction of the late conductor 11 is a short-circuit end and the other end is an open end, the inverted F antenna 17 resonates with an electric wave with a particular frequency f.sub.o depending on the longitudinal length of the plate conductor 11. Hence, the inverted F antenna 17 shown in FIG. 10 acts as a resonator with one short-circuit end and the other open end and possesses a function of an antenna device. Accordingly, a radio wave with a frequency f.sub.o fed to the inverted F antenna 17 via the internal conductor 15 of the coaxial line 14 is resonated by the inverted F antenna 17 to be radiated externally. In this case, the impedance matching of the coaxial line 14 with the inverted F antenna 17 is controlled by the connection position of the internal conductor 15 with the plate conductor 11.
Conventionally, since an antenna device of this kind is constructed by directly connecting the conductor to the inverted F antenna 17, as described above, in order to connect the internal conductor 15 with the plate conductor 11, it is necessary to carry out a soldering operation on the facing surface of the plate conductor 11 to the ground conductor 12 or to provide a through-hole in the plate conductor 11 and the soldering thereon, and thus it is troublesome to manufacture it.
Further, since the impedance matching of the input impedance of the inverted F antenna 17 to the characteristic impedance of the coaxial line 14 is controlled by the connection position of the internal conductor 15 with the plate conductor 11, in order to perform the impedance matching, the connection position of the internal conductor 15 with the plate conductor 11 is unequivocally determined. Hence, the restriction arises in arranging the parts, and the wiring of the coaxial line 14 becomes difficult.
FIG. 11 shows a conventional filter antenna device, as disclosed in "RF Branching System for the Amps Mobile Telephone Equipment", I. YOSHIDA et al., 29th IEEE Vehicular Technology Conference IEEE Catalog No. 79 CH 1378-9 VT, pp. 178-180, 1979. In FIG. 11, an antenna 20 is connected to a band pass filter 22 through a connection cable 24 and connectors 26.
In this case, since the band pass filter 22 is designed so as usually to pass a wave of only a transmission and receiving frequency band, when the antenna device using this conventional filter is used for receiving, the wave within the receiving frequency band received by the antenna 20 is passed through the band pass filter 22 to input to a receiver. However, an unwanted wave outside the receiving frequency band is stopped by the band pass filter 22 and can not be input to the receiver. On the other hand, when the antenna device using this conventional filter is used for transmitting, only the wave within the transmission frequency band is led from the antenna 20 and the unwanted wave outside the transmission frequency band is stopped by the band pass filter 22. Hence, only the wave within the transmission frequency band can be emitted from the antenna 20.
As described above, the antenna device using the filter 22, as shown in FIG. 11, has a function to receive or transmit only the wave within the necessary frequency band.
However, in the conventional antenna device having the structure described above, the connection cable 24 and the connectors 26 for connecting the antenna 20 with the band pass filter 22 are required. Thus, the device is enlarged and the loss is increased. Further, since the antenna 20 and the band pass filter 22 independently function or operate, it is necessary to provide a matching portion for performing the impedance matching of the antenna 20, and the antenna 20 is enlarged. Also, since the antenna 20 constitutes a one stage resonator, it is difficult to perform the impedance matching over a wide band.
In the conventional antenna device, when the inverted F antenna 17 is used as the antenna 20, these problems of the necessity of the matching portion provision and the difficulty of the wide band impedance matching become particularly troublesome.